Page 41 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

INTRODUCTION TO MACHINERY PRINCIPLES

2.14 CHAPTER TWO

They also have a lower cost than samarium-cobalt (SmCo) magnets. Their machining
characteristics, strength, and hardness are similar to those of iron and steel. Figure
2.11 shows a comparison of the NdFeB magnet characteristics with those of other
common magnets. The energy product [product of B in gauss (G) and H in oersteds
(Oe)] and the permeance ratio (ratio of B/H) are also shown on these figures.
Permanent magnets are most efficient when operated at conditions that result in max-
imum energy product. The permeance ratios are useful in designing magnetic circuits.
The flux density B and field intensity H are used to designate the coordinates of the
d d
demagnetization curve.

PRODUCTION OF INDUCED FORCE ON A WIRE

A magnetic field induces a force on a current-carrying conductor within the field (Fig.
2.12). The force induced on the conductor is given by
F i (l B)
The direction of the force is given by the right-hand rule. If the index finger of the right
hand points in the direction of vector l, and the middle finger points in the direction of the
flux density vector B, the thumb will point in the direction of the resultant force on the wire.
The magnitude of the force is
F ilB sin
where is the angle between vector l and vector B.

FIGURE 2.11 Demagnetization curves of certain permanent magnets.

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